Can a woman hold a pidyon haben for her son? The particular situations I’m thinking of are when the father has died or is not Jewish, and so is not available to perform the ceremony. Is it not done until the child’s Bar Mitzvah, or can the (Jewish) mother do it in the father’s stead?

The Shulchan Aruch stupilates that i such a case the obligation of pidyon passes to the child, and therefor should be done at or immediately after the bar mitzvah when the boy is obligated in mitzvot. The Avnei Nezer (Yoreh Deah 395) Is of the opinion that if the mother or anyone else performs pidyon haben it is valid (apparently in the father's absense) but does not clarify if he thinks this should be done when the father is not around, instead of waiting. Presumably, It would be better to wait since tne one doing the pidyon would not be fullfilling any halachic obligation. The conclusion: Better to wait but if the mother or anyone else performed a pidyon, the act has been performed.